Motorcycles often utilize a caliper-type disc brake system to slow or stop the rotation of a wheel, since this type of brake system tends to be lighter weight and provide superior stopping performance in various weather conditions as compared to other types of braking systems. In a typical disc brake system, a disc configured for co-rotation with a wheel rotates on a plane parallel to the plane of the wheel, with the disc passing between two opposing brake calipers with brake pads configured to simultaneously come into contact with opposing sides of the disc when activated to slow or stop the rotation of the disc, and in turn, the wheel. The disc functions optimally when its surfaces are maintained to be as planar as possible, which encourages good mating contact with the brake pads. Thus, protecting the disc from debris or impacts that may cause damage and bend the surface of the rotor is important for performance. Protecting the disc is more problematic for a motorcycle than for an automobile, due to exposure of the disc, as explained below.
Automobile disc brake systems are more advantageously positioned on a common axis with and alongside an axle on the inner side of a wheel between the automobile frame and the wheel. In this position, the disc itself is substantially protected from external damage from rocks and other damaging debris due to the surrounding wheel assembly components such as the wheel, the automobile undercarriage, wheel suspension components, and often a fender or other covering of the wheel and wheel assembly as an extension of the automobile body.
In contrast, a motorcycle disc brake system is open to the exterior and thus is more vulnerable to damage from approaching objects, and from contact with the ground. This exposed configuration is problematic for road-use motorcycles that generally are used on paved surfaces, due to debris that may be contacted when riding on such surfaces. Further, such an exposed configuration is even more problematic for off-road motorcycles which are often operated off road in a more debris laden environment, under more extreme riding conditions in which such debris is flung up with greater force and density, and in which occasionally the motorcycle including its braking system discs may come in contact with the ground.